THE MOVEMENTS OF SHORE ANIMALS 173 



attachment of the suspensory ligament from the tail to the 

 head." The following littoral forms have been observed by 

 Colgan to have this habit of suspending themselves by means 

 of slime threads : Runcina hancockt, Limapontia nigra, 

 Doto coronata, ^olis f arrant, Molis drummondi, Skenea 

 planorhis, Rissoa striata, R. parva, R. cingillus, Modiolaria 

 discors. 



Having now dealt with the actual mechanism of locomo- 

 tion in a number of typical cases, we may turn to a considera- 

 tion of the movements of shore animals in the wide sense. 

 As examples of such movements we have the migrations of 

 varying extent and distinctness in relation to seasonal 

 changes, and in connection with reproduction. To these 

 we may also add the vertical movements of many forms in 

 relation to the tides. Finally, there remains for considera- 

 tion the question as to what extent, if any, there is developed 

 in shore animals a topographical or " homing " sense. 



The Migrations o£ Shore Animals. — Owing to the slow- 

 ness of locomotion in most shore species, such migrations 

 are not likely to be of great extent, and as, so far, hardly 

 any one appears to have observed the distribution of the 

 members of a particular shore association over an extended 

 period, the data relating to such movements are very scanty. 

 Sorby (1901), however, has published some interesting 

 observations on the changes which have taken place, over 

 an extended period, in the distribution of the fauna of the 

 Essex coast (see Chap. HI). He remarks on the extraordi- 

 narily local distribution of some animals, and thinks the exact 

 whereabouts of a particular form may shift from year to 

 year owing to a partial migration. Closer observation, 

 however, is required to determine whether these local 

 changes are due to migration or to the disappearance of the 

 original forms as a result of adverse circumstances and the 

 reinfiltration of a new generation in a more suitable spot. 

 A considerable amount of knowledge, however, has been 

 obtained regarding the seasonal movements of such forms 

 as crabs, prawns, and lobsters, because in addition to their 



